I'd love to see a mechanic where mana points are just "here's how much magic you can control" and you can go beyond that if you want to but crazy unpredictable things will happen and you will almost certainly die.
In the old SNES game Paladin's Quest, HP and MP share an identical pool. You could either hit with your weapon or use magic, depending on which would cost you more. Magic comes at a PRICE!:-P
Mage the awakening (2e) has a concept of Reaches. As in overreaching. You can throw more than your safe amount of reaches on a spell to get it to be bigger, longer, more complicated... but you'll have increasing odds of causing a Problem. And every time you make that check in a scene, it gets harder to pass safely.
The Wheel of Time rpg was 90% D&D3, but it had a fun twist on the casting mechanics that let you try to go beyond your spell slots at the risk of losing all your casting.